Few planetary systems have measured mutual inclinations, and even fewer are found to be non-coplanar. Observing the gravitational interactions between exoplanets is an effective tool to detect non-transiting companions to transiting planets. Evidence of these interactions can manifest in the light curve through transit timing variations (TTVs) and transit duration variations (TDVs). Here, through analysis of Kepler photometry and joint TTV-TDV modelling, we confirm the detection of KOI-134 b, a transiting planet with mass and size similar to Jupiter on a period of ~67 days, and find that it exhibits high TTVs (20-h amplitude) and significant TDVs. We explain these signals with the presence of an innermost non-transiting planet in 2:1 resonance with KOI-134 b. KOI-134 c has a mass M =0.22 0−0.011+0.010MJup and a moderately high mutual inclination with KOI-134 b of imut=15 .4−2.5+2 .8∘. Moreover, the inclination variations of KOI-134 b are so large that the planet is predicted to stop transiting in about 100 years. This system architecture cannot be easily explained by any one formation mechanism, with other dynamical effects needed to excite the planets' mutual inclination while still preserving their resonance.