“The Lost Tribe” is episode eleven of season five of Stargate: Atlantis.
In the previous episode, “First Contact”, Drs. McKay and Jackson had found a secret lab hidden on Atlantis, accidentally activating a subspace signal which caused some unknown aliens to attack Atlantis and steal a device, as well as kidnapping the pair. The alien forced them to activate the device, which destroyed any Wraith ship which entered hyperspace. It also caused any activated stargate to explode. As a result, Todd the Wraith thought he had been betrayed and seized the Daedalus and Atlantis’s stargate exploded, destroying the control room.
It turns out the unknown attackers are actually Asgard, but ones separate from those who finally died out in “Unending” – a splinter faction who separated from the others, as they felt that experimenting on humans would solve their problems. Todd is taking the Daedalus to the outpost to destroy the device, which will incidentally kill Rodney and Daniel, whilst Colonel Sheppard is hitching a ride on a Traveller ship (the Travelers being met in “Travelers”) to get there first, to save Daniel and Rodney, and then destroy the device.
Quite a bit of action. Todd escapes, but it’s possible he may bear a grudge. The discovered Asgard are highly likely to hold one.