The first episode where we learn of the ancient alliance of four races - although the only one identified in this episode is the Asgard that we learned about in the last episode, Thor's Hammer. Ernest Littlefield, who went through the gate in early experiments in the 1940s, and who was Catherine Langford's fiancé, has been living alone on the planet Heliopolis for the past fifty years. During this time he hypothesised that the place was a meeting point for four peoples, and found a display showing 146 chemical elements (Catherine states that 111 were known to Earth scientists, but by this point Copernicum, the 112th had actually been discovered. The current periodic table shows 118 elements). Daniel realises that this was a 'universal language' used between the 'four great races'. Unfortunately they have to leave before he can study it in detail, but Capt. Carter is said to be building a computer model of it based on Dr. Littlefield's notes.
This is quite a nice episode because it brings back Catherine Langford, who first brought Daniel into the Stargate programme in the movie. And, the keen-eyed amongst you may have noticed the young Ernest Littlefield was played by Paul McGillion (Dr. Carson Beckett in Stargate Atlantis). His 'death', as she knew it (actually his disappearance through the stargate) was the reason she pushed so hard for the stargate programme to be revived. It's nice to see the two characters reunited even though we barely know either of them. As far as plot goes, though, this mostly just sets the story for future arcs and episodes.