Adam was captured by Zarkon which Voltron got the signal so while the others distracted the soldiers Shiro was planning to make it a quick in and out mission not knowing who the prisoner was.

While inside the base Shiro moved swiftly and effectively around the base going to where the prisoner was captured and when he made it to the cell and actually looked inside Shiro felt his heart stop seeing who it was after all these years Shiro was looking at Adam and Adam was looking at Shiro.