Open this link: https://www.101computing.net/frequency-analysis/ 

In the Frequency Analysis text box, delete what's already there and paste the following text:

NM NU I MWAMT ASNGHWUIDDE IPJSXKDHBYHB, MTIM I UNSYDH OIS NS RXUUHUUNXS XV I YXXB VXWMASH OAUM CH NS KISM XV I KNVH.

TXKHGHW DNMMDH JSXKS MTH VHHDNSYU XW GNHKU XV UAPT I OIS OIE CH XS TNU VNWUM HSMHWNSY I SHNYTCXAWTXXB, MTNU MWAMT NU UX KHDD VNQHB NS MTH ONSBU XV MTH UAWWXASBNSY VIONDNHU, MTIM TH NU PXSUNBHWHB IU MTH WNYTMVAD RWXRHWME XV UXOH XSH XW XMTHW XV MTHNW BIAYTMHWU.

Click 'Start Frequency Analysis' and looking at the histogram provided, and the patterns of the texts (single letter words, two letter words, consecutive repeated letters), substitute letters and click 'start substitution'. You'll see the letter you've inputted as the key being substituted into the paragraph. What's the plaintext?