We present the results of optical , near-infrared , and mid-infrared observations of M101 OT2015-1 ( PSN J14021678+5426205 ) , a luminous red transient in the Pinwheel galaxy ( M101 ) , spanning a total of 16 years . The lightcurve showed two distinct peaks with absolute magnitudes M _ { r } \leq - 12.4 and M _ { r } \simeq - 12 , on 2014 November 11 and 2015 February 17 , respectively . The spectral energy distributions during the second maximum show a cool outburst temperature of \approx 3700 K and low expansion velocities ( \approx - 300 km s ^ { -1 } ) for the H I , Ca II , Ba II and K I lines . From archival data spanning 15 to 8 years before the outburst , we find a single source consistent with the optically discovered transient which we attribute to being the progenitor ; it has properties consistent with being an F-type yellow supergiant with L \sim 8.7 \times 10 ^ { 4 } L _ { \odot } , T _ { eff } \approx 7000 K and an estimated mass of M 1 = 18 \pm 1 M _ { \odot } . This star has likely just finished the H burning phase in the core , started expanding , and is now crossing the Hertzsprung gap . Based on the combination of observed properties , we argue that the progenitor is a binary system , with the more evolved system overfilling the Roche lobe . Comparison with binary evolution models suggests that the outburst was an extremely rare phenomenon , likely associated with the ejection of the common envelope . The initial mass of the binary progenitor system fills the gap between the merger candidates V838 Mon ( 5 - 10 M _ { \odot } ) and NGC 4490-OT ( 30 M _ { \odot } ) .