As Daenerys prepares to wage war for the Iron Throne on Game of Thrones, she is shoring up support from several different areas — her armies, Tyrion Lannister, Houses Tyrell and Martell, and of course, Varys, the Master of Whispers, who has advised several recent rulers of the Seven Kingdoms. But can she trust Varys? Daenerys isn't so sure, as evidenced by her questioning of him in the episode, "Stormborn."
Daenerys wants to know why she should trust a person who not only helped Robert Baratheon take the throne from her father, Aerys II (aka the Mad King), but who also sold her into slavery, sent Jorah Mormont to spy on her, and helped Robert's bid to have her assassinated in season one.
If you'll recall, in season one, Varys and Illyrio Mopatis seemingly "sell" Daenerys to the Dothraki, but they're actually trying to protect the Targaryen heirs from King Robert. The idea is that in exchange for Daenerys marrying Khal Drogo, the Dothraki will give Dany's brother Viserys an army with which to reclaim the Iron Throne. However, Varys goes along with King Robert's plan to assassinate Daenerys in order to protect himself.
In the episode "The Wolf and the Lion," Robert hears of Daenerys's alliance with the Dothraki via her marriage to Khal Drogo (and also that she is now carrying Khal Drogo's son), so he orders Dany to be killed to stop her from invading Westeros with her Dothraki horde. Varys helps carry out the assassination attempt after Eddard Stark refuses to play a part in killing an innocent pregnant woman (and then resigns as Hand of the King).
Then, in the episode "You Win or You Die," a wine seller attempts to poison Daenerys in the markets of Vaes Dothrak. His attempt is thwarted by Jorah Mormont, who realizes what is about to happen just in time to save Daenerys from drinking the poison. In doing so, Jorah sacrifices his pardon — which would have allowed him to return to Westeros after being exiled for selling poachers into slavery — to save Daenerys's life.
It's a tricky line Varys walks, since he does support whichever ruler he sees as best for the people — but he also must act in his own self-interest at times. Still, Daenerys decides to trust him, promising Varys that if he betrays her, she'll burn him alive.