“The Torment of Tantalus” is episode ten of season one of Stargate SG-1.
Whilst Daniel is trawling through some recently declassified files released by the Pentagon regarding the stargate, he discovers video evidence that the stargate had been previously activated. In 1945, a team working on the stargate managed to get it to work, and someone went through, never to return. The destination wasn’t Abydos.
Daniel goes to see Catherine Langford, who originally recruited him into the stargate programme (back in the Stargate film), whose father was running the tests. The man who went through was her fiancée, Ernest.
Daniel has worked out the address of Ernest’s destination, and SG-1 goes through the stargate, accompanied by Catherine, to see if he is still alive. He is, but unfortunately the Dial Home Device (or DHD as it tends to be called; something that is missing from Earth’s stargate) is broken, and the stargate is in a ruined castle which is falling over the edge of a cliff.
The castle was the meeting point for four alien races, including the Asgard (encountered in the previous episode, “Thor’s Hammer”, for the first time), and there is a wealth of knowledge there, but without a means of getting home, it is valueless. The knowledge that is really needed is how to get the stargate to work.