“Echoes” is episode twelve of season three of Stargate: Atlantis.
Previously, in the season two episode, “Grace Under Pressure”, Rodney McKay was in a Jumper that crashed into the sea. Whilst there he came across a whale-like creature which helped the others find him. When the same creature appears at Atlantis, Rodney and Colonel Sheppard go and investigate it.
Meanwhile, Teyla is seeing apparitions, which she thinks might be partially Ascended Ancients, or ghosts, who are trying to communicate but are incomprehensible. Then others start seeing them too.
This not entirely a follow on episode to “Grace Under Pressure”; it is mostly considered so because of the re-appearance of the whale creature, and would not make a lot of sense without seeing the earlier episode.
“Echoes” is effectively a standalone episode, rather than being linked to the main Wraith storyline. The two plots are actually intertwined, and this does work quite well as a separate story. The overall plot is similar to others, with McKay and Sheppard saving the day, with the help of the usual cast.