Of Tuor and his Coming to Gondolin is the first chapter of Part One of Unfinished Tales.
It tells (as it is said in the title) of Tuor, a mortal, getting into Gondolin. The story began in Dor-lómin. Rían, wife of Huor, had just given birth to a son, who she named Tuor. When Rían learned of Huor's death during the Nirnaeth Arnoediad (Battle of Unnumbered Tears), she went into the wilderness to weep for him, and was never seen again. Tuor was left with the Ñoldor of Dor-lómin, and he grew up with them.
When he was sixteen years of age he left to seek Turgon, the King of Gondolin. He was able to find one of Ulmo's rivers and follow it down to the Great Sea. He tarried there a while, but after some swans showed him a sign that he had stayed too long, he went south. There he found an old, ruinous road, which led to Vinyamar, Turgon's old home. Soon thereafter, Ulmo himself appeared to Tuor and requested that he find Turgon. Tuor then found Voronwë, a mariner that had once sought the havens of Círdan. He was originally from Gondolin, and he promised to lead Tuor there. After a long journey they found the first of the Seven Gates of Gondolin. They were led through the seven gates in order: Gate of Wood, Gate of Stone, Gate of Bronze, Gate of Writhen Iron, Gate of Silver, Gate of Gold, and Gate of Steel. Finally they entered Gondolin, and there the chapter ends.