Bug: Cannot plot spells from exile (e.g. wrenn's resolve)
I cast Wrenn's Resolve on turn 2 (https://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=607221)
Wrenn's Resolve states "Exile the top two cards of your library. Until the end of your next turn, you may play those cards." and it exiled Slickshot Show-Off (https://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=655087) and another spell.
Slickshot has the plot ability. During turn 3 I wanted to plot Slickshot. However, MTG Arena did not give me the option to plot Slickshot like normal; it just cast it. Neither ruling texts on gatherer for either card state that spells can't be plotted from exile.

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DAC169#34537 commented
The rulings on Plot say otherwise. No bug.
Plot abilities are written “Plot [cost],” which means “Any time you have priority during your main phase while the stack is empty, you may pay [cost] and exile this card from your hand. It becomes plotted.”