MacKay is rescued from the collapsed building, but Sheppard and Ronon are trapped deeper. The collapse has sent a warning to Michael, and he arrives back in his Wraith battleship. Then the Daedalus arrives. Much beaming and battling occurs. The team rescue Tayla and her newborn baby from Michael's ship, which is then destroyed. Carter returns to Earth to witness the extraction of the last Baal clone symbiont (as seen in Continuum ): when she arrives, she is informed that the IOA has relieved her of command and put Woolsey in her place.
Woolsey arrives at Atlantis. Dr Keller thinks she might have found a treatment for Carson's organ failure, but isn't sure. Woolsey says her only options are to try it out, or give up, as she won't be able to get any surer. She tries it; it works. Which is just as well, because the team were infected in Michael's lab, and Dr Keller starts sprouting tendrils: a Wraith ship is growing, using her brain as its intelligence. The race is on to find a solution before it consumes the entire city. Carson comes up with a phage that destroys it, and saves Keller. Woolsey is mortified: in his first three days of command he has violated several regulations.
Ronon is captured by his old friend, now Wraith-worshipper, Tyre, who has been outcast by the Wraith since he let Ronon and the team escape before [ 4.03 Reunion ]. Tyre hands Ronon over to the Wraith, hoping to be reinstated as a reward, but is left for dead. The Wraith start turning Ronon. Meanwhile, the Atlantis team capture Tyre, and remove the Wraith virus from him, a painful withdrawal experience. He then leads them in a rescue of Ronon, and dies blowing up the Wraith lab as the others escape. Ronon has to go through an equally painful withdrawal process: it remains to be seen how much his experience has affected him. Tayla decides to resume active service.
A deserted Daedalus appears above Atlantis, and the team goes to investigate. The log says that the crew abandoned ship, but it's from a captain Sheppard doesn't recognise. Suddenly the ship jumps to a parallel universe. McKay discovers the jump drive is stuck, flicking them to a new universe at regular intervals. They go through a series of perils before he finds how to reverse it, at which point they traverse the perils in reverse before arriving home again.
Atlantis gets infected with a strange computer problem -- which claims to be Elizabeth Weir. She says she is the consciousness of the original, left behind with the Replicators [ 4.02 Lifeline ], who fled with other Replicators who wanted to ascend. After they found the Replicator home world destroyed [ 4.11. Be All My Sins Remember'd ], they felt safe to try ascension experiments. But these just left them stranded, bodiless, in subspace. Weir and the others use the Atlantis facilities to make themselves Replicator bodies, in order to then make safe human bodies. But one of them rebels. Weir kills him, and realises the others are not safe, so to protect Atlantis, she tricks them all into going through a stargate that opens into empty space.
Rodney McKay is infected with an inoperable bran parasite that is causing him to slowly lose his mental faculties. His sister Jeannie is brought from Earth to say goodbye to him before he dies.
Sheppard, Beckett, and a team of Marines investigate one of Michael's abandoned labs. They find a group experimental subjects in stasis. When a villager releases them in an attempt to find his missing wife, it all turns into Night of the Living Dead .
Dr Keller has perfected the treatment that will allow Wraith to eat non-human food. Atlantis contacts the "friendly" Wraith Todd [ 4.18 The Kindred ] , who agrees to a meeting. He claims that Tayla will need to pretend to be a Wraith Queen to push the agenda. The he kills the high Queen, putting Tayla in her place. Tayla returns to Atlantis, effectively giving him control of a Hive. They all wonder if this was his plan all along.
Dr Keller is kidnapped by a Runner to save a sick child. Ronon and McKay must find her before the Wraith do.
The Wraith Todd agrees to trial the new drug [ 5.08 The Queen ] . Wolsey, Keller and Ronon go with the Daedelus to a Wraith ship. Meanwhile, Dr Jackson arrives at Atlantis to find Janus' hidden lab [ 1.15 Before I Sleep ] . He does, which sets off a subspace transmitter. A group of strangers arrive, abduct Jackson and McKay, and take a device from the lab. They go to Janus' secret Ancient base, and demand Jackson and McKay engage the device. They realise it is a Wraith ship subspace disruptor, that has a bad side-effect. Forced under pain of death to engage it, they do, destroying two of Todd's Wraith ships. Todd assumes treachery, and takes over the Daedalus. The device's side effect explodes the Stargate on Atlantis...
Jackson and McKay plead with their abductors to turn off the device, as it is exploding Stargates. Their captors are intransigent, and when Jackson pushes, reveal themselves to be Asgard stranded in the Pegasus galaxy, hiding out from the Wraith, determined to destroy them. Todd takes Daedelus to Atlantis, and when he learns of the real culprits, demands their location, and races off to destroy them. Sheppard takes a lift on a traveller's ship [ 4.05 Travellers ] , who came after their Gate was destroyed, to try to get their first and rescue his colleagues. Ronon sabotages Daedelus, delaying it temporarily. When it gets to the base, the Wraith leave, having set the ship locked on a collision course with the base. Sheppard arrives, and manages to open a subspace workhole, saving the ship. The Asgard depart, with the key device. McKay and Jackson manage to sabotage the device; Jackson is badly injured, but they are both rescued by the Daedelus. Woolsey orders the base destroyed, so there can be no repeat of the problem.
Dr Beckett is treating some survivors of the Hoffman plague who have relocated to a different planet. The Wraith arrive, and demand the villagers turn over these outsiders to them, or be destroyed. The team have to work out how to save everybody, while the villagers debate whether to turn them over. Beckett and McKay are captured by the Wraith, who demand they work on an antidote to the Hoffman toxin. Beckett discovers he is infected, and fools the Wraith commander into trying to feed off him, who promptly dies. Eventually, the villagers and outsiders escape through the Stargate.
A "clips" episode. The team are arrested and charged with causing the deaths of 2 million Pegasus Galaxy humans by their reckless actions awakening the Wraith and the Replicators. Woolsey intervenes, and argues their case. He "bribes" one of the Coalition judges with promises of more help against the Wraith, and the team are found not guilty.
"Mission report. Michael invaded Atlantis, tried to blow it up. We stopped him. End of report."
[Michael appears to be really dead this time, having fallen (well, been pushed) from a great height. But I'd only be convinced he was dead if they'd retrieved the body, burnt it, dissolved the ashes in acid, thrown half the remnants into the sun, and the other half into a black hole. Then he'd still come back, from a parallel universe...]
A representative of the IOA comes to evaluate Wolsey's performance during his probationary period, but he is distracted by an attractive new doctor. Sheppard is escorting some scientists on the mainland, but is captured and tortured by a Genii commander that he thought he had killed. McKay and Zelenka uncover a strange undersea probe, and try to analyse it. Eventually, much of this turns out to be manipulation by the probe's AI.
[A nice set of hallucinations: I spotted the obvious ones, but didn't expect the final revelation at the very end.]
McKay invites Keller on a date, to a physics conference. There a rival announce he has a cure for global warming: pump the heat through a space-time bridge to an alternate universe, and that he will demonstrate it. McKay recognises this as one of his ideas, that doesn't work, and tries to warn them, but this is interpreted as jealousy. The device is switched on , the conference centre gets colder, then they find they can't switch it off. McKay eventually comes up with a solution, but let's his rival implement it and take the glory while he dashes off to save Keller from freezing to death.
Todd's hive ship arrives at Atlantis, and sends a garbled message. The team goes to investigate, and discovers all the crew sick, in stasis. The gene therapy [ 5.10 First Contact ] hasn't worked, and the ship is sick too. They barely escape with their lives, aided by an ailing Todd, as the hive crash lands on the planet. Sheppard lets Todd go, having promised to do so to get his aid in landing the ship. "I will remember this" vows Todd.
Thief Neeva Casol finds an Ancient memory transfer device, and ends up swapping her mind with Dr Keller's. She finds herself on Atlantis, but is quickly caught after stabbing Zelenka and acting strangely. Meanwhile Keller, in Casol's body, is sentenced to death for murder. Just as the axe is about to fall, she is rescued by Casol's accomplices, but after one is killed and Keller/Casol shot by the villagers, the other realises something is wrong, and threatens to kill her. The Atlantis team rush to the rescue, and Ronan destroys the device, reversing the swap, just in time.
On an alternate Earth, Detective John Sheppard is tracking down a serial killer who seems to be sucking the life out of people. He discovers that it is an alien -- a Wraith trying to signal the location of Earth to its hive. He stops it just as it sends the signal, leaving it too weak to reach the Pegasus Galaxy, at least, in this universe...
A recovered Todd [ 5.17 Infection ] contacts Atlantis to warn them that a Hive ship has worked out how to use a ZPM to power it, which would make it virtually unbeatable. Daedalus goes to destroy the ship before it can complete its installation, but is badly damaged, even by the partially upgraded Hive. The ship then picks up a signal, and leaves -- it is the location of Earth, sent from the parallel reality [ 5.19 Vegas ] . Sheppard gates to Earth to use the Antarctic chair -- but the Hive arrives and destroys it first. Sheppard then prepares to nuke the Hive. Meanwhile, Beckett flies Atlantis to Earth to defend it from the Hive ship. The team manage to gate onto the Hive, team up with Sheppard, and nuke it. Atlantis lands on Earth, damaged, but shielded.