Within the ruins of the legendary city of Atlantis is buried a secret base left by the Ancients, originators of the Stargate. A new team of explorers travel to the distant galaxy where Atlantis is located, where a primitive human civilization is threatened by a sinister enemy.
The Stargate crew are where we left them, suspended in space with only 24 hours of life support remaining. How will they survive? (S4, ep 1)
It's a race against time to find the missing city of Atlantis as the team embarks on a daring infiltration to steal a ZPM from the Replicators. (S4, ep 2)
Ronon is reunited with a group of his fellow Satedans and is forced to decide where his allegiance lies, with the people from his old civilisation or with Atlantis. (S4, ep 3)
Stargate delves into the past when Sheppard comes across a crystalline species similar to one that Jack O'Neill encountered 10 years ago. (S4, ep 4)
An unfamiliar race, known as the Travelers, hunts down the Atlantis expedition after hearing of the team's notorious ancient techniques used to fight the Wraith. (S4, ep 5)
A deadly infection threatens the Atlantis expedition when the team contracts a virus that causes memory loss. Ronan and Teyla must find a cure. (S4, ep 6)
Teyla and Dr Keller visit New Athos in search of Teyla's people, who have gone missing from their planet. Trouble strikes when primitive warriors hunt the pair down. (S4, ep 7)
An Oracle visits Atlantis, warning the team that Wraith hive ships are heading in their direction. He also exposes a deep secret Teyla has yet to reveal. (S4, ep 8)
McKay and his sister are kidnapped on Earth by a man in need of their expertise. The rest of the team must enlist the help of an NID agent to rescue them. (S4, ep 9)
The Atlantis team's worst fears are realised when a mysterious probe crashes into the city, and the crew are convinced the Replicators have found them. (S4, ep 10)
The Atlantis team strives to eliminate the Replicators once and for all, but with time in short supply they may well have a crisis on their hands. (S4, ep 11)
The team successfully salvage a partially damaged Hive ship and in the process retrieve some critical information that may help in their war against the Wraith. (S4, ep 12)
When an alarm sounds throughout the city, initiating a quarantine lock down, the Atlantis team find themselves trapped in various sections of the city. (S4, ep 13)
Wanting to maintain diplomatic relations, Sheppard and McKay agree to accompany a precocious young princess on a rite of passage. But unexpected enemies prove a threat. (S4, ep 14)
When Lt Colonel John Sheppard receives word that his father has died, he returns to Earth. But once there, he becomes embroiled in a covert experiment gone awry. (S4, ep 15)
During an off-world mission, Carter, McKay and Dr Keller become trapped in an underground mining chamber, rocked by fierce seismic tremors. How will they escape? (S4, ep 16)
When Ronon comes under review by the IOA, Carter asks Teal'c to mentor him, but the appraisal is put on hold when the Wraith target the Midway station. (S4, ep 17)
The Kindred (Part 1): Dr Keller discovers that a Wraith-killing protein is infecting thousands of humans, and Teyla convinces Sheppard to join the search for her people. (S4, ep 18)
The Kindred (Part 2): The Atlantis team are stunned when an old face returns, and Teyla fights for her people and her life when Wraith Michael takes her prisoner. (S4, ep 19)
Season four comes to a shattering climax when, having failed to locate Teyla, Sheppard returns to Atlantis to find the city abandoned and all systems dead. (S4, ep 20)