A novel wideband vertically polarized planar end-fire antenna based on the first and the second high-order modes of Spoof Surface Plasmon Polaritons (SSPPs) is proposed in this letter. A double-sided SSPP transmission line (TL) is used to produce vertically polarized radiation. By replacing the uniform stub in the conventional SSPP cells with a step-impedance-resonator (SIR) shaped stub, the bandgap between the first and the second high-order modes is eliminated. The double-sided SIR-shaped SSPP cells are tilted to make the maximum radiation point in the end-fire direction. Dielectric covers perforated with air vias are added to the top and bottom surfaces of the SSPP TL to generate end-fire radiation and enhance antenna gain. A grounded coplanar waveguide to double-sided parallel-strip line transition is designed to feed the radiator. A prototype of the proposed antenna has been fabricated and measured. The proposed antenna achieves a fractional bandwidth of 91.6% from 12.9 GHz to 34.6 GHz and a peak gain of 17.3 dBi.