Search your library for an instant or sorcery card, reveal it, then shuffle and put that card on top.
Mystical Tutor is one of the cleanest, cheapest instant/sorcery tutors in blue, and it earns its Game Changer flag for good reason: at you can fetch any answer or wincon at instant speed, end-of-turn or in response to a threat.
The catch is it puts the card on top of your library, so you're paying a card disadvantage and a turn of tempo to draw it next. That makes it best when you have a specific high-impact target — a board wipe, a counterspell, a combo piece (Ad Nauseam, Time Walk effects), or a finisher like Expropriate.
Who wants it: spellslinger decks, combo/control commanders (Kess, Talrand, Niv-Mizzet), and any cEDH-adjacent blue list with a deterministic instant/sorcery win. It also shines with surveil/topdeck synergies and Storm.
Skip it in creature-centric, tokens, or aristocrats decks with few impactful instants/sorceries — there, you're just fetching a single mediocre spell for a card down. Tutors reward having a great target.
The “shuffle and put the card on top” is a single action. If an effect causes the top card of the library to be face up, the second card down is not revealed.
