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The Sc galaxy M 99 in the Virgo Cluster has been strongly affected by recent tidal interactions, responsible for an asymmetric spiral pattern and a high star formation rate (∼ 10 M⊙ yr−1). We studied the galaxy with XMM-Newton, Keck and the Very Large Array (VLA). The inner disk is dominated by hot plasma with a total X-ray luminosity ≈ 1041 erg s−1. At the outskirts of the galaxy, away from the main star-forming regions, there is an ultraluminous X-ray source (ULX) with an X-ray luminosity ≈ 2 × 1040 erg s−1 and a hard spectrum (power-law photon index Γ ≈ 1.7). This source is close to the location where a massive H I cloud appears to be falling onto the M 99 disk at a relative speed > 100 km s−1. The infalling gas may have been stripped from the nearby “dark galaxy” candidate VIRGOHI 21. We speculate there may be a relation between collisional events, infall of metal-poor gas clouds, and ULX formation.